First, do all of your students have an equitable opportunity to audition? How many have instruments? How many have the time and space needed to record? You said you are giving them multiple chances to record, but that still doesn’t account for several other issues that could arise. Are you providing recording equipment and/or recording tutorials so that all students have an equal chance at an audition.

Above all else, is auditioning something you should be concerned with right now? Think of it this way, are you evaluating their playing or their recording ability?

If this is something that you’ve truly planned out and believe to be an absolute necessity, email based on how you would have told them in-person. A roster to the class or individual emails without scores seems acceptable.